Rotaviruses are major causal agents of acute gastroenteritis in man with a world-wide distribution. Enteroviruses have been implicated in infections of the gastrointestinal tract, the respiratory system and in aseptic meningitis. Respiratory syncytial virus (RSV), the parainfluenza viruses and the influenza viruses have been shown to be the major causal agents of serious pediatric diseases such as croup, bronchiolitis and pneumonia, while the influenza viruses are the major cause of serious febrile respiratory tract illness in adults. Hepatitis A virus is the cause of hepatitis A, one of the major infectious diseases of the liver.
Rotavirus infections occur world-wide and are responsible for a large proportion of severe, life-threatening, often fatal diarrheal disease. Most rotavirus disease is caused by the four major human rotavirus serotypes but other serotypes are being discovered continuously. The latter are usually restricted in geographic distribution, but they could become a larger problem at any time. World-wide, the human rotaviruses are the major etiologic agents of serious acute gastroenteritis in infants and young children and on occasion can cause debilitating diarrhea in adults. It has been estimated by the World Health Organization that rotaviruses are responsible for one million fatal diarrheal illnesses each year in infants and young children in developing countries.
The most important cause of serious viral respiratory illness in children is respiratory syncytial virus (RSV). The parainfluenza viruses, influenza viruses, and adenoviruses are also important in this regard. Especially in the case of respiratory syncytial virus (RSV), of which there are two major subgroups A and B, immunity does not appear to be long-lasting and re-infections can and do occur with high frequency during infancy, through the pre-school period, and throughout adult life. The parainfluenza viruses, of which there are four major types--1, 2, 3 and 4--have been implicated as important causes of croup, bronchiolitis and pneumonia in infants and young children. These viruses are second only to RSV as a cause of severe viral respiratory tract disease. In adults, the influenza viruses are a major cause of mortality in older persons with underlying acute or chronic cardiac or pulmonary disease. The serious systemic disease manifestations of influenza virus infections are well known and the frequent antigenic shifts in serotype [A/H1N1, A/H3N2 and B] necessitate changes in the composition of the strains included in the yearly vaccine preparations.
The enteroviruses, which are a subgroup of picornaviruses consisting of polioviruses, Coxsackie viruses and echoviruses, have been shown to cause a broad spectrum of illnesses. These illnesses include paralytic disease, encephalitis, aseptic meningitis, pleurodynia, exanthems, and pericarditis with some of the infections resulting in debilitating sequelae. Hepatitis A virus, also a picornavirus, is a major cause of sporadic as well as epidemic hepatitis.
According to convention, semi-continuous cell systems which are diploid and have a finite longevity in terms of the number of passages that can be achieved in the laboratory are designated as cell strains, whereas continuous cell systems that are aneuploid and can be passaged indefinitely in the laboratory (i.e., are immortal) are designated as cell lines. A limiting factor in the development of vaccines for combating the maladies caused by a number of these infectious agents has been the lack of availability of an acceptable cell strain or line capable of supporting the growth of these viruses to a concentration suitable for use in vaccine production. Convenient and susceptible cell strains and/or lines that can be cultured using relatively high split ratios (i.e., the preparation of multiple tissue culture vessels from a single culture) and which can be approved for human use by the Center for Biologics Evaluation and Research (CBER) of the Food and Drug Administration (FDA) are needed to enable and facilitate vaccine development for certain medically important viruses that are uncontrolled currently.
Semi-continuous or continuous cell culture systems capable of supporting the growth of hybrid rotaviruses, hereafter designated human x animal rotavirus reassortants, have included the simian cell strain, FRhL-2, and the simian cell lines, CV-1 and Vero. Live virus vaccines produced in these cells have received CBER, FDA approval for Phase I and II studies. Henceforth, we will refer to these cell strains or lines as certified cell strains or lines to distinguish them from cell strains or lines that have been licensed for production of human vaccines. To date, only two semi-continuous cell culture systems, WI-38 and MRC-5, both human fetal diploid cell strains, have been licensed for use in virus vaccines. These cell strains will be referred to as licensed cell strains. Currently, there are no licensed continuous cell lines.
The above mentioned certified cell strain or lines (FRhL-2, CV-1 and Vero) are limited in their ability to support the growth of completely homologous human rotaviruses, i.e., rotaviruses that derive each of their 11 RNA gene segments from a human rotavirus. In addition, the FRhL-2 cell strain, with a maximum 1:3 split ratio capability, is limited in its ability to support the growth of important RSV. The CV-1 cells, with a maximum 1:4 split ratio capability, have not supported the growth of many viral agents to a level satisfactory for vaccine production. The Vero cells, with a split ratio of 1:6-1:10, are limited in their ability to support the growth of a number of enteroviruses and fail to support the efficient growth of completely homologous human rotaviruses.
Naturally occurring strains of hepatitis A virus (HAV), a picornavirus distantly related to the enteroviruses, do not grow well in any cell type during primary isolation and must be adapted to growth in cells of primate origin before the level of virus replication required for vaccine development and production can be achieved. Inactivated whole-virus HAV vaccines grown in fetal human fibroblast MRC-5 cells have been developed, but the low split ratio of 1:2 required by these cells, the low viral titers achieved, and the prolonged cultivation time of up to two weeks necessary for maximum yield of viral antigen make such vaccines expensive. Growth of HAV in simian CV-1, FRhK4 (a fetal rhesus monkey kidney cell line), FRhK-6 (another fetal rhesus monkey kidney cell line) and Vero cells has been variable depending on the strain and passage level of virus, but generally is suboptimal. Best growth has been obtained in a cloned cell line, designated clone 11-1, derived from FRhK4 cells, but these cells contain bovine papilloma virus sequences and thus, are not suitable for vaccine development.
Candidate live attenuated and inactivated HAV vaccines have been developed by adaptation of the virus to growth in primary African green monkey (AGMK) cells but such vaccines are not economically feasible because of the extreme difficulty of obtaining sufficient primary AGMK cells that are free of extraneous viral agents of monkey origin. Live attenuated and inactivated HAV vaccine candidates have been adapted to growth in MRC-5 cells, a human fetal diploid cell strain, but such adaptation has consistently led, respectively, to over-attenuation of the live attenuated virus for humans and a relatively sparse yield of inactivated virus vaccine.
Thus, there remains a need for a cell strain or line capable of supporting the efficient growth of a large number of human viral pathogens such as the human rotaviruses, enteroviruses, HAV, and respiratory viruses of major medical importance including RSV, influenza viruses and parainfluenza viruses. Such a cell strain or line would facilitate the development and production of commercially useful and effective vaccines. There is a pressing need for the development of vaccines against these viruses in order to prevent severe viral diseases of infants, children, adults and the elderly.
Therefore, the availability of a single cell strain or line capable of supporting the efficient replication of those viral agents responsible for a significant number of childhood and adult diseases would provide a significant advancement in the formulation and production of multivalent vaccines.